The use of group VIB metals and combinations thereof with group VIII metals as active components for the preparation of catalysts having very high activity in hydrorefining reactions has been well known for many years. At the present time, the most active catalysts known to the art combine with the aforementioned metals a catalyst support having a refractory oxide base, e.g., alumina, silica or silica-alumina.
Supported hydrorefining catalysts are most often sold in the "oxide" form, that is, after having been subjected to calcination at an elevated temperature in the presence of air, such that the active metals are chemically linked to oxygen. However, hydrorefining of hydrocarbon cuts containing sulfur impurities results in hydrodesulfuration by the hydrogen, with hydrogen sulfide as the product. Consequently, the stable forms of the active metals contained in the catalysts during use, in the reaction atmosphere and under the operating conditions, are the sulfides. It is also well known to the art that it is the association of the metal sulfides of the group VIB and VIII metals which constitutes the active component of these catalysts, such that their industrial use always begins by a presulfuration step, which determines the level of activity which will result during use.
The precise crystallochemical nature of the sulfides which form on the supported catalysts is not known and cannot be directly determined by radio-crystallagraphic methods or by electron microscopy, as attested to, for example, in the scientific literature, e.g., Bull Soc. Chim. Belges, 90, No. 12, (1981) and Transactions of the Fourth International Congress on the Chemistry and Applications of Molybdenum, held on August 8-16, 1982, at Golden, Colo., USA. In particular, the presence of a disulfide of a group VIII metal has never been observable in a supported sulfided hydrorefining catalyst.
The prior art has also known non-supported catalysts associating group VIB and group VIII metal sulfides. For example, the catalyst denoted 3076, sold by I. G. Farben Co., is known and described in the CIOS report, Volume XXXII-107, pages 115-124, published in 1945. This catalyst associates nickel monosulfide NiS with tungsten disulfide WS.sub.2 in a molar ratio of 2:1. The sulfides are obtained by treatment of a mixture of nickel carbonate + tungstic acid at a temperature of 420.degree.-460.degree. C. with a mixture of H.sub.2 +H.sub.2 S. U.S. Pat. No. 2,394,739 discloses a method of preparing a hydrogenation catalyst composed equally of a mixture of nickel monosulfide and tungsten disulfide, according to which an aqueous solution of nickel nitrate is added to an ammoniacal solution of ammonium thiotungstate and ammonium sulfide in excess. The nickel monosulfide precipitates, after which the solution is acidified to pH 2 and tungsten trisulfide precipitates. It is also possible to add an acidic solution of nickel nitrate to the thiotungstate solution. In all cases, the precipitate is reduced in the presence of hydrogen after drying.
Pratt et al studied a series of sulfided catalysts based on nickel and molybdenum which they prepared: J. Catalysis, 66, 82-92 (1980). Their method of preparation comprises a continuous coprecipitation starting with aqueous solutions of nickel nitrate and ammonium heptamolybdate, in the absence of sulfur. Drying and calcination of the precipitate in air for four hours at 400.degree. C. produces a finely divided mixture of nickel and molybdenum oxides. After sulfuration in situ by 20% by volume H.sub.2 S in H.sub.2 during four hours at 400.degree. C., these mixtures were tested for their catalytic activity in hydrogenolysis of thiophene. After the tests, the catalysts were characterized. One of the most active catalysts, corresponding to an atomic ratio (nickel/nickel+molybdenum) of 0.73, had a specific surface of about 50 m.sup.2 /g, a total stoichiometry of about 1 atom of sulfur per metal atom, and the presence of MOS.sub.2, NiS.sub.2 and NiS in the catalyst was detected by X-ray diffraction.
U.S. Pat. No. 4,303,634 describes the preparation of a non-supported catalyst of molybdenum disulfide by thermal decomposition in an inert atmosphere of an ammonium salt of molybdenum sulfide obtained by the reaction between molybdenum trioxide and ammonium sulfide. U.S. Pat. No. 4,243,554 describes an analogous method for obtaining an analogous catalyst, starting with an ammonium thiomolybdate salt of well defined composition. Thus, these patents describe methods of preparing catalysts which only contain molybdenum.
Topsoe et al, J. Catalysis, 68, 433-452 (1981), and Candia et al, Bull. Soc. Chim. Belges, 90 (12), 1225-1252 (1981), have described a method of preparing non-supported catalysts comprising mixed sulfides of cobalt and molybdenum, the method being called "HSP", standing for "Homogeneous Sulfide Precipitation", according to which an aqueous solution of cobalt nitrate and ammonium heptamolybdate, having a predetermined atomic ratio Co/Mo, is added to a solution of 20% by weight of ammonium sulfide at a pH higher than 10, maintained between about 60.degree.-80.degree. C. under vigorous agitation, the added solution having also been brought to the same temperature. The reaction is effected in air and the mixture is maintained at temperature with stirring until all of the water is evaporated. The resultant precipitate, which is the prescursor of the final catalyst, is then treated with a stream of 2-6% by volume of H.sub.2 S and H.sub.2, at a temperature of between 320.degree.-400.degree. C., for four hours, for a final sulfuration, then cooled under nitrogen and conditioned under argon. Catalysts prepared by the HSP method, characterized by Candia et al by X-ray diffraction, were shown to consist of a single homogeneous phase analogous to molybdenite. Another variant of the HSP method was described by Breysse et al, 184th ASC National Meeting, Div. Pet. Chem., Kansas City, Sept. 12-17, 1982. According to this variant, it is a dilute solution of ammonium sulfide which is added slowly to a very dilute solution of ammonium heptamolybdate and cobalt nitrate having a predetermined atomic ratio Co/Mo. An inert atmosphere is not used. After evaporation of water, the recovered solid is resulfided under 15 volume per cent H.sub.2 S in H.sub.2, for four hours, at 400.degree. C.